A common method for the reduction of flicker on CRT display screens at readily achievable refresh rates is to use `interlace` where a single frame of the image is displayed as two fields, the first containing odd raster lines and the second even raster lines. The image of the first field is reinforced by the image of the second field and, upon rapid refresh, the finite persistence of the CRT phosphor produces a stable image. By this means, flicker frequency is increased from the frame to the field frequency and is consequently less obtrusive.
In order to fully realize this advantage of interlace, the displayed image should be equally distributed over the two interlaced fields. In the case of data displays, for example, where an image is represented on the screen by a plurality of individual visible picture elements (pels), the effect of interlace can be diminished due to the non-random nature of the images. This leads to an unequal distribution of pels between the fields resulting in increased perception of flicker, the frequency of which appears to be at the frame frequency.
Various techniques have been employed to overcome this problem with varying degrees of success. Thus, one method of equalizing the energy content of the two interlaced fields is that known as `double-dotting`. Here the information content of each field is duplicated so that every horizontal stroke forming a character in the displayed image is produced by two individual raster scan lines, one from each of the two interlace fields. The disadvantage of this system of flicker reduction is that either the displayed image must be limited to relatively large characters because of the pel duplication in the vertical scan direction or suffer from a loss of resolution. Neither of these constraints are acceptable to modern day visual display unit (VDU) users who demand the capability to display small characters without a reduction of resolution.
An alternative approach to the double-dotting method of flicker reduction is described in IBM Technical Disclosure Bulletin Vol. 21, No. 4, September 1978 at page 1673 entitled `Reduction of Flicker in Interlaced CRT Data Displays` by B. F. Dowden. (IBM is a registered trademark of International Business Machines Corporation). The technique described in this article to ensure a more even distribution of pels between the two interlaced fields is to select a character set in which the uppercase characters, for example, have an even number of pels in the vertical strokes. The disadvantage of this technique is that VDU users may not wish to be constrained to use a particular character set especially where this may require changes to be made to a character generator ROS.
Yet another approach to solving the problem is described in IBM Technical Disclosure Bulletin, Vol. 21, No. 4, September 1978 at page 1675 entitled `Flicker Reduction in Interlaced CRT Data Displays` by J. H. Boal and B. F. Dowden. In this case, there is no restriction on the choice of character design, but the display control system functions to ensure that the display of alternate rows of characters is `started` in alternate interlaced fields. This method has the merit that it places no restriction on the character set which may be designed for optimum character discrimination or have some particular stylistic attributes. A disadvantage in this case is that the operational constraint on the display system which can result in non-uniform line spacing. Furthermore, both of the latter references describe methods which although serving to reduce the flicker are essentially only partial solutions.